1. Field of the Invention
The present invention relates generally to optical amplifiers. More particularly, the invention relates to an optical amplifier employing dual wavelength pumping to reduce the steady state population in the intermediate termination energy level by repopulating the metastable energy level. The power and/or wavelength of the second pump may be adjusted to alter the shape of the amplifier gain spectrum.
2. Technical Background
Optical amplifiers increase the amplitude of an optical wave through a process known as stimulated emission in which a photon, supplied as the input signal, induces higher energy level electrons within an optical material to undergo a transition to a lower energy level. In the process, the material emits a photon with the same frequency, direction and polarization as the initial photon. These two photons can, in turn, serve to stimulate the emission of two additional photons, and so forth. The result is coherent light amplification. Stimulated emission occurs when the photon energy is nearly equal to the atomic transition energy difference. For this reason, the process produces amplification in one or more bands of frequencies determined by the atomic line width.
While there are a number of different optical amplifier configurations in use today, the optical fiber configuration is quite popular, particularly for optical communications applications. The optical fiber amplifier typically consists of an optical material such as glass, combined with a rare earth dopant and configured as an optical waveguide. Rare-earth-doped silica fibers are popular today, in part because they offer the advantages of single-mode guided wave optics. Optical fiber amplifiers can be made to operate over a broad range of wavelengths, dictated by the atomic properties of the host and rare earth dopant.
The phenomenal growth in communication technology and information technology has fueled considerable interest in finding new optical fiber materials that will increase signal channel bandwidth and allow engineers to exploit new frequency bands.
One difficulty encountered in the rare-earth materials has to do with the materials"" inherent low multi-phonon decay rates. It is desirable to have a long lifetime in the metastable energy level because it aids stimulated emission. However, it can be undesirable if the particular material exhibits a similarly long lifetime in an intermediate termination energy level.
By way of example, Praseodymium doped chalcogenide glass possesses a 1G4 (metastable energy level) lifetime of around 300 xcexcs with a radiative quantum efficiency of greater than 50% typically. However, a further consequence of this low multi-phonon decay rate is the relatively long lifetime for the 3H5 energy level (the intermediate termination energy level for the 1.3 xcexcm transition). A typical value for the lifetime of the 3H5 energy level is in the range of 100 xcexcs in chalcogenide glasses.
In an optical amplifier fabricated from this material, pump energy supplied at 1020 nm excites atoms in the material from the 3H4 ground energy state to the 1G4 metastable state. The input optical signal interacts with the material in this excited state to produce photons by stimulated emission, and thereby causing electrons in the metastable state to fall to an intermediate termination energy level of 3H5. While the electrons are in the intermediate 3H5 energy level, they are no longer available for use. Only after they decay back to the ground energy level 3H4 can they be re-pumped to the metastable energy level 1G4 where they can take part in further stimulated emission processes.
Thus, the long 3H5 energy level lifetime has a detrimental effect when the material is used as a fiber amplifier. The 3H5 population lowers the gain efficiency of the amplifier and tends to shift the peak operating wavelength away from the desired wavelength.
The aforementioned difficulty is not unique to Praseodymium (Pr3+) doped chalcogenide glass; rather, it exists in other low and intermediate phonon energy glasses as well, including any one from chalcogenide, halide, tellurite, germanate, aluminate and gallate glass fibers doped with either Thulium (Tm3+) or Holmium (Ho3+).
It is an object of the present invention to provide an optical amplifier and a pumping technique that overcomes the difficulties associated with long intermediate termination energy level lifetimes exhibited by certain rare earth doped materials.
According to one aspect of the invention, the optical amplifier comprises an optical waveguide having an optical host that contains a rare earth dopant. The host and dopant define a ground energy state. The amplifier further includes a first pump optically coupled to the waveguide. This first pump supplies optical energy into the waveguide at a first wavelength. The first pump establishes a metastable energy state above the ground energy state. An input, coupled to the optical waveguide, introduces an optical signal to be amplified, where amplification is produced by stimulated emission of photons from the metastable energy state. This establishes a termination energy state below the first metastable energy state and above the ground energy state. The optical amplifier further comprises a second pump optically coupled to the waveguide that supplies optical energy to the waveguide at a second wavelength. The second pump repopulates the first metastable energy state by depopulating the termination energy state.
The resulting optical amplifier configuration is suitable for use with a number of different optical fiber materials, including Praseodymium doped chalcogenide glass fibers, and in particular sulfide glasses, Thulium or Holmium doped glasses such as chalcogenides, halides, tellurites, germanates, aluminates and gallates.
For a more complete understanding of the invention, its objects and advantages, refer to the following specification and to the accompanying drawings. Additional features and advantages of the invention are set forth in the detailed description which follows.
It should be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various features and embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.